Spatial radionuclides distribution was studied in recent estuarine sediments in the Krka River Estuary. Sediment samples were analyzed for 40K and 232Th by gamma-spectrometry. Activities of 40K were found in the range of 18–457 Bq kg–1 and activities of 232Th in range of 1.9–29.4 Bq kg–1. Distribution of 232Th activities follows that of 40K, with correlation coefficients higher than 0.97 in all analyzed samples from the Krka River estuary. Spatial distribution of natural radionuclides (40K and 232Th) in recent estuarine and marine sedimentation area can be a useful tool for recognizing the possible input of terrigenous material, as well as for fast locating of the area where such material is predominantly settled.U radu je proučavana prostorna raspodjela radionuklida u recentnim sedimentima estuarija rijeke Krke. Uzorci sedimenta analizirani su tijekom protekle dvije godine i aktivnost 40K i 232Th određena je gamaspektrometrijski. Izmjerene aktivnosti za 40K bile su između 18–457 Bq kg–1, a za 232Th izme|u 1.9–29.4 Bq kg–1. Raspodjela aktivnosti 232Th prati raspodjelu aktivnosti 40K, i njihov korelacijski faktor vi{i je od 0,97 u svim analiziranim uzorcima. Prostorna raspodjela prirodnih radionuklida (40K i 232Th) u recentnim estuarijskim i morskim sedimentima pod značajnim je utjecajem unosa terigenog materijala i može biti koristan ključ za prepoznavanje mogućeg izvora materijala kao i boljeg određivanja mjesta najvećeg taloženja tog materijala

Delko Barišić

Neven Cukrov

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Spatial Distribution of 40K and 232Th in Recent Sedimentsof the Krka River Estuary*Neven Cukrov** and Delko Bari{i}Division for Marine and Environmental Research, Ru|er Bo{kovi} Institute, Bijeni~ka c. 54,P.O. Box 180, 10002 Zagreb, CroatiaRECEIVED MAY 27, 2005; REVISED OCTOBER 7, 2005; ACCEPTED JANUARY 2, 2006Spatial radionuclides distribution was studied in recent estuarine sediments in the Krka RiverEstuary. Sediment samples were analyzed for 40K and 232Th by gamma-spectrometry. Activ-ities of 40K were found in the range of 18–457 Bq kg–1 and activities of 232Th in range of1.9–29.4 Bq kg–1. Distribution of 232Th activities follows that of 40K, with correlation coeffi-cients higher than 0.97 in all analyzed samples from the Krka River estuary. Spatial distribu-tion of natural radionuclides (40K and 232Th) in recent estuarine and marine sedimentation areacan be a useful tool for recognizing the possible input of terrigenous material, as well as forfast locating of the area where such material is predominantly settled.KeywordsKrka River Estuarysedimentsradionuclide distributiongamma-spectrometry40K232Th* Dedicated to the memory of the late Professor Marko Branica.** Author to whom correspondence should be addressed. (E-mail: ncukrov@irb.hr)CROATICA CHEMICA ACTACCACAA 79 (1) 115¿118 (2006)ISSN-0011-1643CCA-3071Original Scientific PaperINTRODUCTIONRiver Krka is located in the central part of the north-eastcoast of the Adriatic Sea and drains mostly carbonateterrains of karst regions of Croatia. Its estuary was form-ed during the Holocene transgression. Stretching from theSkradinski Buk waterfalls (calc-tufa barrier, height 46 m)through the Prokljan Lake to the St. Ante Channel, theestuary has a total length of 22 km. The hydrogeologicdrainage area of the Krka River is approximately 2427km2. The estuary bottom gradually deepens from 2 m,below the waterfalls, to 42 m in front of Fort Sv. Nikola.It is a typical example of a stratified estuary with a fresh-brackish surface layer moving seawards and a bottomseawater layer, as counter-current, moving upward. Clas-tic material input in the Krka River estuary is small.2 Themain input of terrigenous material in the Krka River es-tuary originates from the very small Gudu~a River inflow-ing into the Prokljan Lake downstream the Krka River.The Krka River carries larger quantities of fresh-water(on average 55 m3/s) than the Gudu~a River (on averageless than 1 m3/s). However, a number of calc tufa barri-ers along the Krka River, upstream of the town of Skradin,significantly reduce suspended material transport.1,2,3 Thelargest settlement in the Krka estuary region is [ibenik,located in the lower part of the estuary. Sedimentationrate in the estuary was found to be very low, less than0.5 mm in the Prokljan Lake.1Contents of naturally occurring radionuclides in dif-ferent types of the recent Adriatic Sea sediments have notbeen systematically studied. However, some data on theactivities of radionuclides in sediment were publishedelsewhere.3,4,5 This work gives a contribution to theknowledge of spatial radionuclide distribution in the KrkaRiver Estuary.EXPERIMENTALSampling Sites and ProceduresSediment samples were collected during 2003 and 2004 by ascuba diver using hand-driven Plexiglas corers ( = 175 mm).Starting points of the investigation area were the SkradinskiBuk waterfalls (KE-1) and Fort Sv. Nikola (KE-17) was theend point. Additionally, sampling was performed at two lo-cations: the first in the Adriatic Sea near the island ofZlarin (Z-1) and the second in the Prokljan Lake (KE-18)near the small Gudu~a River mouth (Figure 1).Sampling positions were located by GPS, model GarminGPSMap 72 (Kansas City, USA) with accuracy of  5 m.At most locations, the first 15 cm of the bottom sedimentwere sampled, but at some locations (KE-1, KE-2, KE-4,NPK-1 and Z-1) only the first 5 cm were taken. After sam-pling, sediment colons were separated in 3 parts (0–5 cm,5–10 cm, 10–15 cm) and frozen at –18 °C until analysis.Equipment and Experimental ProcedurePrior to gamma-spectrometry measurements, sediment sam-ples were thawed at room temperature and dried at 106 °Cfor 24 hours until constant weight. Dried samples were placedinto counting vessels of known geometry, sealed and storedfor at least 4 weeks to allow ingrowth of gaseous 222Rn (3.8day half-life) and its short-lived decay products to equili-brate with long-lived 226Ra precursor in the sample. At theend of the ingrowth period, the samples were counted on amultichannel gamma spectrometer (HPGe detector joinedto a 8192 channel analyzer Canberra, Meriden, USA). Thesystem was calibrated using standards supplied by AmershamInternational (Buckinghamshire, UK) and International Ato-mic Energy Agency reference materials (marine and streamsediments IAEA-306, IAEA-313 and IAEA-314). Countingtime was 80,000 seconds and the recorded spectra were an-alyzed on a PC using the Canberra GENIE 2K software.Activities of 40K were calculated from the 1460.75 keV-peakwhile 228Ra activities were determined from the 911.1 keV-peak of its 228Ac progeny. Because of the relatively shorthalf-life of 228Ra (t1/2 = 5.75 years) and thorium conserva-tive behavior, numerous authors equalize measured 228Raactivities with 232Th activities in natural samples,6,7,8 evenin cases of possible radium migration in extreme condi-tions.9 The 309.3 Bq kg–1 of 40K activity corresponds to 1% total potassium while 4.06 Bq kg–1 of the 232Th activitiescorresponds to a concentration of 1 mg/kg of thorium in theground.10RESULTSSediments with the highest 40K activity (457.1  10.4 Bqkg–1) were found in the Prokljan Lake in front of the smallGudu~a River mouth (KE-18) in the sediment layer be-tween 10 and 15 cm depth, while the lowest activities(18.0  4.4 Bq kg–1) were found close to the SkradinskiBuk waterfalls (KE-1) and (20.5  4.3 Bq kg–1) near theisland of Zlarin (Z-1) in the sediment layer between thesurface and 5 cm depth (Figure 2).At the same locations (KE-1 and Z-1), the highest(29.4  1.4 Bq kg–1) and the lowest (1.9  0.5 and 3.5 0.6 Bq kg–1) 232Th activities were also recorded (Figure2).In all sediment layers (0–5 cm, 5–10 cm and 10–15cm), an increasing trend of 40K and 232Th activities wasfound, rising from the open sea to the Prokljan Lake en-trance (KE-5). Only one exception was found at samplingsite KE-9, close to the Zaton village (Figure 1). From the116 N. CUKROV AND D. BARI[I]Croat. Chem. Acta 79 (1) 115¿118 (2006)Figure 1. Map of the sampling area.Figure 2. Plots of spatial distribution of 40K and 232Th activities ex-pressed in Bq/kg dry weight.KE-5 sampling site (Figure 1), the opposite trend of 40Kand 232Th activities in sediments was recorded towardsthe Skradinski Buk waterfalls (KE-4, KE-3, KE-2 andKE-1), and a continuous increase towards the Gudu~aRiver mouth (KE-18) where sediments contain the high-est 40K and 232Th activities (Figure 2).At sampling sites KE-7, KE-8, KE-9, KE-10 andKE-18, concentrations of 232Th and 40K slightly increas-ed from the sediment surface to deeper layers (Figure 2).Such behavior is most significant in samples of layersbetween 0–5 cm and 5–10 cm.In all the sampled sediment layers (0–5 cm, 5–10 cmand 10–15 cm) the correlation coefficient of the spatialdistribution of 40K activities with those of 232Th, washigher than 0.97, indicating very good correlation be-tween 40K and 232Th concentrations (Figure 2). Correla-tion was calculated using the »CORREL« function inMicrosoft Excel 2003 between all 40K and all 232Th data.DISCUSSIONThe present work was aimed at stimulating research onnatural radionuclides in recent stream, estuarine and ma-rine sedimentation areas as a possible sediment sourceindicator in recent sediment accumulation.The Krka River estuary functions as a big pump –fresh water flows on the surface towards the open seaestablishing an opposite deep flow of salt water up-stream in the estuary. Such circulation makes marinecarbonate sedimentation influence the sediment compo-sition even many kilometers inward the estuary.Low activities of 40K (20.5  4.3 Bq kg–1) and of232Th (3.5  0.6 Bq kg–1) at Z-1 probably correspond tothe characteristic values4,5 of the sand and silts spreadalong the Croatian coast representing the marine car-bonate sedimentation. At KE-10 and KE-11 samplingpoints, the marine carbonate sedimentation influencestill dominates. On the other hand, low activities of 40K(18.0  4.4 Bq kg–1) and 232Th (1.9  0.5 Bq kg–1) atKE-1 most likely correspond to tufa 11 with 40K activityof 12 Bq kg–1 and 232Th of 2.5 Bq kg–1 originating fromthe Krka River input.Increased activities of 40K and 232Th are found inprofiles from the Skradinski Buk waterfalls towards Pro-kljan Lake, reflecting the relative decreasing contributionof tufa material. However, the highest activities werefound at the KE-18 sampling point, predominantly underthe Gudu~a River material transportation influence. TheGudu~a River drainage area consists mainly of the Eoceneflysch material, so high concentrations of potassium andmoderate concentrations of thorium in sediments are ex-pected. Hence, the spatial distribution of 40K and 232Thactivities in Prokljan Lake recent sediments additionallyconfirms that the main input of terrigenous material tothe Krka River estuary originates from the small Gudu~aRiver, flowing into the Prokljan Lake downstream theKrka River. It seems that most of the terrigenous mate-rial is quickly sedimented in the Prokljan Lake in the vi-cinity of the Gudu~a River mouth (Figure 1).At the KE-5 sampling point, recent sediment containsa higher portion of terrigenous material, also eroded fromthe Eocene flysh terrains. It seems that even at this pointthe influence of the Gudu~a River input significantlyprevails over the Krka River input and/or marine carbo-nate sedimentation. Here, fresh water mostly transportsmaterial from the inland canyon configuration (Figure 1)and interrupts the water pump that could otherwise trans-port recent marine carbonate sediment into that area up-stream the estuary. At the KE-9 sampling point, similarlocal influence of terrigenous material (eroded from Eo-cene flysch terrains lying west of the Zaton village) wasobserved as well. However, carbonate marine sedimen-tation, with minor terrigenous influence, generally pre-vails in the downstream direction.In deeper sediment layers (5–10 cm, 10–15 cm), thedistribution of 40K and 232Th activities was somewhat dif-ferent than in the surface layer. This probably indicates adifferent sedimentation regime in the recent past.CONCLUSIONSThe distribution of 232Th activities follows that of 40K,and their correlation coefficient is higher than 0.97 in allthe analyzed samples from the Krka River estuary. Onthe basis of the prevailing origin of accumulated mate-rial, four main sedimentation areas are recognized: (i)from the open sea to the KE-10 sampling point wherecarbonate marine sedimentation prevails; (ii) from theKE-10 to the KE-6 sampling point where settled mate-rial is a mixture of two main origins: marine carbonatesand terrigenous flysch material transported by theGudu~a River; (iii) upstream of KE-3 where most of thesettled material is carried by the Krka River, and (iv) theProkljan Lake (including the bay of Zaton situated to thewest of KE-9) where recent sediment consists mainly ofterrigenous flysch material transported by the Gudu~a Ri-ver (and occasional local creak in the case of the Zatonbay). It seems that the spatial distribution of natural radio-nuclides in recent sedimentation areas is under signifi-cant influence of terrigenous material input, and could bea useful tool for recognizing the possible sources of ma-terial as well as for better locating of the areas wherethat material is predominantly deposited.REFERENCES1. M. Jura~i} and E. Prohi},Geol. Vjesnik 44 (1991) 195–200.2. E. Prohi} and M. Jura~i}, Environ. Geol. Water Sci. 13(1989) 145–151.40K AND 232Th IN SEDIMENTS OF THE KRKA RIVER ESTUARY 117Croat. Chem. Acta 79 (1) 115¿118 (2006)3. N. Cukrov, D. Bari{i}, and M. Branica, Proc. 37th CIESMCongress, Barcelona, 7–11 June 2004, 2004, p. 20.4. D. Bari{i}, A. Verta~nik, S. Luli}, G. Mihel~i}, I. Sondi, M.Jura~i}, E. Prohi}, and R. Crmari}, Radiological character-istics of recent marine sediments of the Adriatic sea, Inter-national Seminar on the Use of Isotope Techniques in MarineEnvironmental Studies. Athens, Greece, IAEA-SR-194/8,1996.5. D. Bari{i}, A. Verta~nik, S. Luli}, G. Mihel~i}, I. Sondi, M.Jura~i}, E. Prohi}, and R. Crmari}, XXXVth CIESM, Con-gress Proc., Vol. 35 (1), 1998, pp. 228–229.6. R. Carpenter, M. L. Peterson, J. T. Bennett, and B. L. K.Somayajulu, Geochim. Cosmochim. Acta 48 (1984) 1949–1963.7. A. S. Murray and M. J. Aitken, Int. J. Appl. Radiat. Isot. 39(1988) 145–158.8. D. J. Greeman, A. W. Rose, and W. A. Jester, GeophysicalRes. Lett. 17 (1990) 833–836.9. Z. Z. Sheng, J. Rad. Nucl. Chem.-Art., 134 (1989) 293–298.10. D. Bari{i}, J. Environ. Radioactivity 31 (1996) 51–70.11. S. Fran~i{kovi}-Bilinski, D. Bari{i}, A. Verta~nik, H. Bi-linski, and E. Prohi}, Facies 50 (2004) 183–193.SA@ETAKProstorna raspodjela 40K i 232Th u recentnim sedimentima estuarija rijeke KrkeNeven Cukrov i Delko Bari{i}U radu je prou~avana prostorna raspodjela radionuklida u recentnim sedimentima estuarija rijeke Krke.Uzorci sedimenta analizirani su tijekom protekle dvije godine i aktivnost 40K i 232Th odre|ena je gamaspektro-metrijski. Izmjerene aktivnosti za 40K bile su izme|u 18–457 Bq kg–1, a za 232Th izme|u 1.9–29.4 Bq kg–1.Raspodjela aktivnosti 232Th prati raspodjelu aktivnosti 40K, i njihov korelacijski faktor vi{i je od 0,97 u svimanaliziranim uzorcima. Prostorna raspodjela prirodnih radionuklida (40K i 232Th) u recentnim estuarijskim imorskim sedimentima pod zna~ajnim je utjecajem unosa terigenog materijala i mo`e biti koristan klju~ za pre-poznavanje mogu}eg izvora materijala kao i boljeg odre|ivanja mjesta najve}eg talo`enja tog materijala.118 N. CUKROV AND D. BARI[I]Croat. Chem. Acta 79 (1) 115¿118 (2006)

Prostorna raspodjela 40K i 232Th u recentnim sedimentima estuarija rijeke Krke

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